The present invention relates to a device and method for accurately measuring the amount of metabolic energy expended by an individual during periods of motion, such as walking or running. The device and method of this invention utilize a predictive mathematical relationship to correlate the time of foot contact with a rigid surface, and the weight of the individual, to calculate the metabolic energy expenditure.
Much time and effort in this country and throughout the world is devoted to physical fitness and exercise. The demands for more effective forms of exercise have increased as a result of the time limitations placed on daily activities, and the tendency toward a more sedentary (and less healthy) lifestyle. In addition, issues of individual health are of increasing concern to an aging population. This has led to a search for more efficient methods of exercising, which in turn has spurred the development of increasingly complex and expensive exercise equipment. For example, the use of heart rate monitors for routine exercising is not commonplace. Although the benefits and risks of being physically fit are becoming more apparent, current methods and devices are limited by their unreliability, expense and obtrusiveness.
It is known that most of the force exerted by an individual's muscles acts to oppose gravity, with the average vertical force equal to the total weight of the subject. Thus, the total cost of locomotion, that is the total rate of energy expenditure, varies with body weight and the speed of walking or running. In other words, the metabolic cost of walking and running is primarily determined by the cost of supporting body weight and the rate at which this force is generated.
Existing methods and instruments for measuring metabolic energy expenditure are often imprecise and complex to use. Activity diaries are simple but inherently inaccurate, whereas heart rate monitoring and accelerometry require laborious individual calibration to estimate energy expenditure. Without individual calibration, heart rate estimates of energy expenditure are highly inaccurate, and accelerometry can only be used to differentiate activity from inactivity. Even without individual calibration, heart rate is inaccurate at low activity levels, while the nonlinearity of accelerometer output at high rates of energy expenditure results in significant underestimation of energy expenditure during running.
It has been postulated that the rate of metabolic energy expenditure can be estimated as the total body weight divided by the time during each stride that a single foot is in contact with the ground. As the speed of locomotion increases, foot contact time decreases, and the rate of force generation increases. Thus, a good approximation of the metabolic cost of locomotion may be obtained from measurements of total body weight and foot contact time. This has been demonstrated for non-human animal subjects by Kram et al., Nature, 346, 265-267 (1990). Kram et al. concluded that there is a simple inverse relationship between the rate of energy used by an animal for running and the time the foot applies force to the ground during each stride. There is no recognition in Kram et al. as to how this finding could be utilized in a practical manner, or how the finding could be applied to human subjects under conditions of running or walking.
The present invention provides a simplified approach to accurately assess the frequency, duration and intensity of metabolic energy expenditure in humans.